Polyisocyanates have been reacted in the presence of a trimerization catalyst to produce polyisocyanurate foams, in an attempt to produce foams which produce high char resistance to fire and low smoke values. The trimerization of polyisocyanates produce foams of superior fire performance over conventional polyurethane foams. The rigid trimerized polyisocyanurate-type foams produced tend to have higher K-factors than urethane foams, but also are quite brittle and friable in nature. The trimerization of methylene diisocyanate produces a foam of low smoke value, but black in color, and which foam is relatively nonburning. The problems with such methylene diisocyanate trimerized foam are the poor compressive strength and high friability.
In order to correct or to mask these undesirable properties, it has been the practice to incorporate small amounts of urethane polyols, such as sucrose amine, glycols or aromatic amines, in the isocyanate reaction mixture, to produce a modified, urethane polyisocyanurate foam. However, the addition of such urethane polyols leads to degradation in the fire and smoke properties of the modified foam.
One standard, widely accepted, two-stage catalyst system used in industry in the preparation of polyol-modified polyisocyanurate foam comprises the combination of dimethyl aminomethyl phenol known as DMP-30 and potassium octoate (see, for example, U.S. Pat. No. 4,101,465). Modified polyisocyanurate foams have been prepared with the standard catalyst and polyethylene glycol in the presence of a blowing agent and a surfactant. In such foam preparation, the higher the ratio of the polyethylene glycol to isocyanate, the more flexible the foam; however, the higher glycol content reduces the fire and flame properties of the foam. The standard catalyst sytstem used permits some urethane linkages to form before trimerization of the isocyanate. The resulting foam, with higher ratios of polyol to isocyanate, then causes black smoke and a reduction in burning properties, so that flame-retardant agents are added to reduce the flame-spread properties. If low ratios of glycol to isocyanate are employed, often flame-retardant agents may be omitted, but the foam has more rigid and friable properties.
Polyurethane foams have been modified in the past by the addition of certain amounts of melamine powder in the reaction mixture, to confer enhanced flame resistance on the resulting polyurethane foam (see U.S. Pat. No. 4,221,875, issued Sept. 9, 1980). In addition, aromatic amine-containing polyols have been used in the preparation of sprayed, rigid, urethane foam compositions, in order to increase the compressive strength of the foam (such as the use of Thanol R-650X polyol, a product of Texaco Chemical Company). The aromatic amine-containing polyol reportedly has catalytic activity which also reduces the catalyst requirements of the urethane foam system and increases the compressive strength of the foam. However, the employment of such product often, as with the use of other polyols, diminishes or degrades the smoke, flame or nonburning properties of the resulting, more rigid urethane foam.